Defective bone remodeling is the physiopathologic basis of most metabolic bone diseases, including postmenopausal and age- dependent osteoporosis. Bone remodeling is the result of a regulated, sequential activation of osteoclasts and osteoblasts within basic multicellular units, and results in a cyclical succession of bone resorptive and formative phases. In turn, bone formation requires the synchronized activity of osteoblasts within each bone remodeling unit Osteoblasts communicate with each other via gap junctions, transcellular channels which allow exchange of ions and small molecules between adjacent cells. Gap junctions in osteoblasts are formed by two different gap junction proteins, called connexins. One of these, connexin43 (Cx43) mediates "chemical" or "dye" coupling between osteoblasts. The other, connexin45 (Cx45), forms functional gap functions of different molecular permeabilities than those formed by Cx43. Cx43 and Cx45 interact with each other, so that when Cx45 is expressed in cells that normally express Cx43, chemical communication between cells decreases. Importantly, is change in gap junctional permeability is associated with a decreased expression of osteocalcin and bone sialoprotein, two matrix proteins pivotal for bone formation. We hypothesize that gap junctional communication between osteoblasts is critical to their maturation and function as bone forming cells, and that the relative expression of Cx43 and Cx45 regulates osteoblast function via changes in gap junctional communication. Exploiting the partial "dominant negative" action of Cx45 on gap junctions formed by Cx43, we propose to determine the regulatory mechanisms of gap junctional communication on osteoblast gene expression, by mapping the regions of the osteocalcin and bone sialoprotein promoters sensitive to gap junctional communication. The consequences of a decreased communication competence induced b overexpressing Cx45 on the capacity of osteoblasts to produce calcified matrix,will be tested in human osteoblastic models. Finally, osteoblasts activity will be characterized in cells derived from Cx43 null ("knockout") mice, in relation to the type of gap junctional communication existing in these Cx43 null cells. If an adequate degree of gapjunctional communication between osteoblasts is necessary for bone formation, abnormalities of gap junction fi1nction or expression may represent potential novel mechanism at the basis of the osteoblastic "failure" characteristic of postmenopausal and aging osteoporotic bone. Therefore, the results of this project will define a fundamental mechanism that regulates osteoblast function in health and disease, and lay the basis for novel therapeutic approaches for the management of osteoporotic syndromes.